JP2004303591A

JP2004303591A - Nonaqueous electrolyte secondary battery

Info

Publication number: JP2004303591A
Application number: JP2003095915A
Authority: JP
Inventors: Anten Iwami; 安展岩見; Nobumichi Nishida; 伸道西田
Original assignee: Sanyo Electric Co Ltd
Current assignee: Sanyo Electric Co Ltd
Priority date: 2003-03-31
Filing date: 2003-03-31
Publication date: 2004-10-28

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery composed by improving degradation of capacity restoration performance after low-temperature charging when only heterogeneous element-added lithium cobaltate is used. <P>SOLUTION: This nonaqueous electrolyte secondary battery uses lithium cobaltate as a positive electrode active material, and carbon as a negative electrode active material. Lithium cobaltate formed of a mixture of one kind of lithium cobaltate with a heterogeneous element added and another kind of lithium cobaltate without a heterogeneous element added is used for the lithium cobaltate. The mixing ratio of the lithium cobaltate with a heterogeneous element added to the lithium cobaltate without a heterogeneous element added is in the range of 9.5:0.5-6.5:3.5, and preferably 9:1-7:3 in a mass ratio. At least one kind selected from Zr, Ti and F is preferably used as the heterogeneous element. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０００１】
この発明は、異種元素添加コバルト酸リチウム（ＬｉＣｏＯ_２）を正極材料として用いた非水電解質二次電池に関し、特に異種元素添加コバルト酸リチウムのみを使用した場合の低温充電後の容量復帰性能の低下を改良した非水電解質二次電池に関する。
【０００２】
【従来の技術】
携帯型の電子機器の急速な普及に伴い、それに使用される電池への要求仕様は、年々厳しくなり、特に小型・薄型化、高容量でサイクル特性が優れ、性能の安定したものが要求されている。そして、二次電池分野では他の電池に比べて高エネルギー密度であるリチウム非水電解質二次電池が注目され、このリチウム非水電解質二次電池の占める割合は二次電池市場において大きな伸びを示している。
【０００３】
このリチウム非水電解質二次電池は、細長いシート状の銅箔等からなる負極芯体（集電体）の両面にリチウムイオンを吸蔵放出する負極活物質を含む負極合剤を塗布した負極と、細長いシート状のアルミニウム箔等からなる正極芯体の両面にリチウムイオンを吸蔵・放出する正極活物質を含む正極合剤を塗布した正極との間に、微多孔性ポリプロピレンフィルム等からなるセパレータを配置し、負極及び正極をセパレータにより互いに絶縁した状態で円柱状又は楕円形状に巻回した後、負極及び正極の各所定部分にそれぞれ負極タブ及び正極タブを接続し、必要に応じて押し潰して偏平巻回電極体として、その外側を外装で被覆することにより製造されている。
【０００４】
そして、正極材料として、従来から主としてコバルト酸リチウム（ＬｉＣｏＯ_２）が、リチウムに対し４Ｖ以上の電位を示し、高エネルギー密度を有する二次電池が実現できることから使用されているが、コバルトは高価であると共に資源としての存在量が少ないため、このコバルト酸リチウムを非水電解質二次電池の正極材料として使用し続けるには、非水電解質二次電池のさらなる高性能化及び高寿命化が望まれ、特に負荷特性、サイクル特性の向上が望まれている。
【０００５】
一方、コバルト酸リチウムを正極活物質として用いたリチウム非水電解質二次電池の特性向上方法として、コバルト酸リチウムへ異種元素を添加する方法が知られている。例えば、下記特許文献１には正極活物質であるコバルト酸リチウムにジルコニウムを添加することで、高電圧を発生し、かつ優れた充放電特性と保存特性を示す非水電解質二次電池が開示されている。
【０００６】
このジルコニウムを添加したコバルト酸リチウムは、ＬｉＣｏＯ_２粒子の表面が酸化ジルコニウムＺｒＯ_２もしくはリチウムとジルコニウムとの複合酸化物Ｌｉ_２ＺｒＯ_３により覆われることによって安定化され、その結果、高い電位においても電解液の分解反応や結晶破壊を起こすことなく、優れたサイクル特性、保存特性を示す正極活物質が得られることによるものであって、この効果は、単に焼成後のＬｉＣｏＯ_２にジルコニウムもしくはジルコニウムの化合物を混合するだけでは得られず、リチウム塩とコバルト化合物とを混合したものにジルコニウムを添加して焼成することにより得られるものである。
【０００７】
【特許文献１】
特開平４−３１９２６０号公報（特許請求の範囲、段落［０００６］、［０００８］〜［００１１］）
【０００８】
【発明が解決しようとする課題】
本発明者等は、既に異種元素としてジルコニウム（Ｚｒ）のみでなく、チタン（Ｔｉ）及びフッ素（Ｆ）をも含めた中から少なくとも１種を正極活物質として使用することにより、リチウム非水電解質二次電池の負荷特性及びサイクル特性を向上させることができることを見出しているが、異種元素としてＺｒ、Ｔｉ、Ｆから選択された少なくとも１種を添加したコバルト酸リチウムのみを正極活物質に用いたリチウム非水電解質二次電池においては、同時に低温充電後の容量復帰性能の低下を伴うという問題点が存在していた。このような問題点は異種元素を添加しないリチウム酸コバルトには生じない現象である。
【０００９】
そこで発明者等は、種々実験を重ねた結果、異種元素添加コバルト酸リチウムと異種元素を添加しないコバルト酸リチウムを併用すると、異種元素添加コバルト酸リチウムのみを正極活物質として使用した際の低温充電後の容量復帰性能の低下という欠点は異種元素を添加しないコバルト酸リチウムにより補われ、逆に異種元素を添加しないコバルト酸リチウムの負荷特性及びサイクル特性が劣るという欠点は異種元素添加コバルト酸リチウムにより補われ、両者の利点が有効に相乗的に発揮されることを見出し、本発明を完成するに至ったのである。
【００１０】
すなわち、本発明は、異種元素添加コバルト酸リチウムのみを使用した場合の低温充電後の容量復帰性能の低下を改良した非水電解質二次電池を提供することを目的とする。
【００１１】
【課題を解決するための手段】
本発明の上記目的は、以下の態様により解決することができる。本発明の一態様によれば、正極活物質としてコバルト酸リチウム、負極活物質として炭素を用いた非水電解質二次電池において、該コバルト酸リチウムとして、異種元素が添加されたコバルト酸リチウム及び異種元素が添加されていないコバルト酸リチウムの混合物からなるものを用いた非水電解質二次電池が提供される。
【００１２】
係る態様によれば、異種元素添加コバルト酸リチウムのみを正極活物質として使用した際の低温充電後の容量復帰性能の低下という欠点は異種元素を添加しないコバルト酸リチウムにより補われ、逆に異種元素を添加しないコバルト酸リチウムの負荷特性及びサイクル特性に劣るという欠点は異種元素添加コバルト酸リチウムにより補われ、両者の利点が相乗的に発揮されるようになる。
【００１３】
係る態様においては、前記異種元素が添加されたコバルト酸リチウム及び異種元素が添加されていないコバルト酸リチウムの混合比が質量比で９．５：０．５〜６．５：３．５であることが好ましく、より好ましくは９：１〜７：３である。異種元素添加コバルト酸リチウムの割合が９５質量部を超えると、負荷特性及びサイクル特性は良好であるが、低温充電後の容量復帰率が低下するので好ましくない。また、異種元素添加コバルト酸リチウムの割合が６５重量部未満であると低温充電後の容量復帰率は良好となるが、逆に負荷特性及びサイクル特性が劣化するので好ましくない。
【００１４】
さらに、係る態様によれば、前記異種元素がＺｒ、Ｔｉ、Ｆから選択された少なくとも１種であることが好ましい。これらの元素を含有している異種元素添加コバルト酸リチウムを用いればコバルト酸リチウムを正極活物質として用いた非水電解質二次電池の負荷特性及びサイクル特性を有効に向上させることができるようになる。
【００１５】
【作用】
本発明者等の検討した結果によれば、上記の態様により異種元素添加コバルト酸リチウムのみを正極活物質として使用した際の低温充電後の容量復帰率が低下するという問題点が解決される理由は、次のとおりであると思われる。
【００１６】
すなわち、リチウム非水電解質二次電池は、一般に低温において充電を行なうと、その後の電池容量の低下が、室温で充電されたときと比べ大きくなることが知られている。これは、リチウム非水電解質二次電池においては、充電に伴い電気化学的にリチウムイオンが正極のコバルト酸リチウムから脱離し、負極の炭素に挿入される反応が起こっているわけであるが、室温よりも極端に低温の状態において充電されると、正極から脱離したリチウムイオンの一部が負極上にリチウム金属として析出し、以降の充放電反応に関与できなくなる。低温時に負極にリチウム金属が析出する理由は、低温充電時には負極の分極が室温充電時に比べ大きくなり、負極の電位がリチウム金属の析出する電位まで下がってしまうこと、及び、リチウム金属の析出量は充電反応時の負極の分極の程度によって決まるためであると推定される。
【００１７】
異種元素を添加したコバルト酸リチウムを正極に用いると、異種元素無添加のコバルト酸リチウムを正極に用いたときに比べ、低温充電後の容量復帰性能が低下する。したがって、負極の分極は、正極材料の影響を強く受けており、低温における負極の分極は、異種元素添加コバルト酸リチウムを正極に用いたときの方が、異種元素を添加しないコバルト酸リチウムを正極に用いたときより大きいと推定される。
【００１８】
そして、異種元素添加コバルト酸リチウムに、異種元素無添加のコバルト酸リチウムを５〜３５質量部、さらに好ましくは１０〜３０質量部混合すると、この異種元素無添加のコバルト酸リチウムの混合により、負極の低温における分極が緩和され、低温充電時のリチウム析出量が減少し、結果として低温充電後の容量復帰率が向上したものであると推定され、そして、異種元素添加のコバルト酸リチウムに対する異種元素無添加のコバルト酸リチウムの混合比が５質量部より小さい場合、低温充電後の容量復帰性能が向上しなかったのは、十分に分極が緩和されなかったためと推定される。
【００１９】
一方、異種元素無添加のコバルト酸リチウムの混合比が３５質量部を越えると、低温充電後の容量復帰性能の大きな向上はなかったが、負荷特性の低下が確認された。これは、異種元素無添加のコバルト酸リチウムの混合比が３５質量部を超えると、異種元素添加の十分な効果が得られないためと考えられる。したがって、本発明では、負荷特性及びサイクル特性を維持し、低温充電後の容量復帰性能を向上させるためには、異種元素を添加したコバルト酸リチウムと、異種元素を添加していないコバルト酸リチウムの混合比を質量比で９．５：０．５〜６．５：３．５、好ましくは９：１〜７：３の範囲で混合することが必要であると結論付けた。
【００２０】
【発明の実施の形態】
以下、本発明の具体例を実施例及び比較例により詳細に説明する。
（実施例１〜５、比較例１〜２）
［正極の作製］
出発原料として、リチウム源には炭酸リチウム（Ｌｉ_２ＣＯ_３）を用いた。コバルト源には、炭酸コバルト合成時に異種元素としてジルコニウム（Ｚｒ）を正極活物質の総量に対する質量比で１９００ｐｐｍとなるように添加した四酸化コバルト（Ｃｏ_３Ｏ_４）を用いた。これらをＬｉ／Ｃｏのモル比が１になるように秤量後、乳鉢で混合し、これを空気雰囲気下において８５０℃で２０時間焼成し、Ｚｒ添加コバルト酸リチウム（ＬｉＣｏＯ_２）を得た。これを乳鉢で平均粒径約１０μｍまで粉砕し、正極活物質とした。異種元素無添加のＬｉＣｏＯ_２の合成には、Ｚｒを添加していないＣｏ_３Ｏ_４をコバルト源として用いた以外は、Ｚｒ添加ＬｉＣｏＯ_２と同様の方法で作製した。なお、正極活物質の組成は、プラズマ発光分析（ＩｎｄｕｃｔｉｖｅＣｏｕｐｌｅｄＰｌａｓｍａ：ＩＣＰ）により分析した。
【００２１】
このＺｒ添加ＬｉＣｏＯ_２粉末と無添加ＬｉＣｏＯ_２粉末を、所定の混合比で混ぜ、その混合ＬｉＣｏＯ_２粉末が８５質量部、導電剤としての炭素粉末が１０質量部、結着剤としてのポリフッ化ビニリデン粉末が５質量部となるよう混合し、これをＮ−メチルピロリドン（ＮＭＰ）溶液と混合してスラリーを調整した。このスラリーを厚さ２０μｍのアルミニウム製の集電体の両面にドクターブレード法により塗布して活物質層を形成した後、圧縮ローラーを用いて１７０μｍに圧縮、短辺の長さが５５ｍｍで、長辺の長さが５００ｍｍの正極を作製した。
【００２２】
［負極の作製］
天然黒鉛粉末が９５質量部と、ポリフッ化ビリニデン粉末が５質量部となるよう混合し、これをＮＭＰ溶液と混合してスラリーを調整し、このスラリーを厚さ１８μｍの銅製の集電体の片面にドクターブレード法により塗布して活物質層を形成した。その後、圧縮ローラーを用いて１５５μｍに圧縮し、短辺の長さが５７ｍｍ、長辺の長さが５００ｍｍの負極を作製した。
【００２３】
［電解液の作製］
エチレンカーボネートとジエルカーボネートとの等体積混合溶媒に、ＬｉＰＦ_６を１ｍｏｌ／Ｌ溶解して電解液とし、これを電池作製に用いた。
【００２４】
［負荷特性の測定］
２５℃において、各電池を定電流充電（電流１５００ｍＡ、終止電圧４．２Ｖ）−定電圧充電（電圧４．２Ｖ、終止電流３０ｍＡ）−定電流放電（電流４５００ｍＡ、終止電圧２．７５Ｖ）した後、定電流充電（電流１５００ｍＡ、終止電圧４．２Ｖ）−定電圧充電（電圧４．２Ｖ、終止電流３０ｍＡ）−定電流放電（電流４５００ｍＡ、終止電圧２．７５Ｖ）した。負荷特性は、１５００ｍＡ定電流放電容量に対する４５００ｍＡ定電流放電容量の比で評価した。
【００２５】
［低温充電後の容量復帰率の測定］
２５℃において、各電池を定電流充電（電流１５００ｍＡ、終止電圧４．２Ｖ）−定電圧充電（電圧４．２Ｖ、終止電流３０ｍＡ）−定電流放電（電流１５００ｍＡ、終止電圧２．７５Ｖ）した。次に、０℃で定電流充電（電流１５００ｍＡ、終止電圧４．２Ｖ）−定電圧充電（電圧４．２Ｖ、終止電流３０ｍＡ）−定電流放電（電流１５００ｍＡ、終止電圧２．７５Ｖ）を３サイクル繰り返した。最後に、２５℃で定電流充電（電流１５００ｍＡ、終止電圧４．２Ｖ）−定電圧充電（電圧４．２Ｖ、終止電流３０ｍＡ）−定電流放電（電流１５００ｍＡ、終止電圧２．７５Ｖ）とサイクルした。低温充電後の容量復帰率は、初期の２５℃での放電容量に対する、０℃から２５℃に戻した後の放電容量の比で評価した。
【００２６】
［サイクル容量維持率の測定］
２５℃又は４５℃の所定の温度において、各電池を定電流充電（電流１５００ｍＡ、終止電圧４．２Ｖ）−定電圧充電（電圧４．２Ｖ、終止電流３０ｍＡ）−定電流放電（電流１５００ｍＡ、終止電圧２．７５Ｖ）した。これを１サイクル目とした。各電池の１サイクル目の放電容量に対する、５００サイクル目の放電容量の比を容量維持率として評価した。
【００２７】
実施例１〜５及び比較例１〜２として、Ｚｒ添加ＬｉＣｏＯ_２及びＺｒ無添加ＬｉＣｏＯ_２の添加割合を種々変化させてリチウム非水電解質二次電池を作成し、それぞれの電池について負荷特性、低温充電後容量復帰率及び２５℃でのサイクル容量維持率を測定した結果を表１及び図１にまとめて示す。なお、負荷特性及び２５℃サイクル特性は比較例１に係る電池の値を１００とした場合の相対値であり、また、低温充電後の容量復帰率特性は比較例２に係る電池の値を１００とした場合の相対値である。
【表１】

【００２８】
表１及び図１に示すように、Ｚｒ添加ＬｉＣｏＯ_２とＺｒ無添加ＬｉＣｏＯ_２の混合比が９．５：０．５〜６．５：３．５の範囲にあるときに、低温充電後容量復帰率は、無添加ＬｉＣｏＯ_２を混合しないものよりも向上した。なお、混合比を９．５：０．５としたときには、低温充電後容量復帰率の低下が目立ち、混合比を６．５：３．５としたときには、負荷特性の低下が目立った。従って、Ｚｒ添加ＬｉＣｏＯ_２とＺｒ無添加ＬｉＣｏＯ_２混合比は、好ましくは９．５：０．５〜６．５：３．５、さらに好ましくは９：１〜７：３の範囲である。
【００２９】
（実施例６〜１０、比較例３〜４）
実施例６〜１０としては、異種元素としてフッ素（Ｆ）を使用し、まず、実施例１〜３における異種元素としてのジルコニウム（Ｚｒ）に換えてＬｉＦを使用し、フッ素（Ｆ）量が３５０ｐｐｍとなるように使用した以外は実施例１〜３と同様に合成した異種元素添加ＬｉＣｏＯ_２を用い、実施例６〜１０に係る電池を作製した。比較例３〜４としては、ＬｉＦを添加しなかった以外は実施例５〜８と同様にして作成した異種元素無添加のＬｉＣｏＯ_２を作成し、比較例６〜１０の電池を作製した。それぞれの電池について負荷特性、低温充電後容量復帰率及び４５℃でのサイクル容量維持率を測定した。結果を表２及び図２にまとめて示す。なお、負荷特性及び４５℃サイクル特性は比較例３に係る電池の値を１００とした場合の相対値であり、また、低温充電後の容量復帰率特性は比較例４に係る電池の値を１００とした場合の相対値である。
【表２】

【００３０】
表２及び図２に示すように、Ｆ添加ＬｉＣｏＯ_２とＦ無添加ＬｉＣｏＯ_２の混合比が９．５：０．５〜６．５：３．５の範囲にあるときに、低温充電後容量復帰率は、無添加ＬｉＣｏＯ_２を混合しないものよりも向上した。なお、混合比を９．５：０．５としたときには、低温充電後容量復帰率の低下が目立ち、混合比を６．５：３．５としたときには、負荷特性の低下が目立った。従って、Ｆ添加ＬｉＣｏＯ_２とＦ無添加ＬｉＣｏＯ_２混合比は、好ましくは９．５：０．５〜６．５：３．５、さらに好ましくは９：１〜７：３の範囲である。
【００３１】
（実施例１１〜１５、比較例５〜６）
実施例１１〜１５としては、異種元素としてチタン（Ｔｉ）及びフッ素（Ｆ）を同時に用い、まず、実施例１〜３における異種元素としてのジルコニウム（Ｚｒ）に換えてＬｉＦをフッ素（Ｆ）の量が３５０ｐｐｍとなり、また、チタン（Ｔｉ）の量が８００ｐｐｍとなるようにＴｉＯ_２を使用した以外は実施例１〜３と同様に合成した異種元素添加ＬｉＣｏＯ_２を用い、実施例１１〜１５に係る電池を作製した。比較例５〜６としては、ＴｉＯ_２やＬｉＦを添加しなかった以外は実施例１１〜１５と同様にして異種元素無添加のＬｉＣｏＯ_２を作成し、比較例５〜６の電池を作製した。それぞれの電池について負荷特性、低温充電後容量復帰率及び２５℃でのサイクル容量維持率を測定した。結果を表３及び図３にまとめて示す。なお、負荷特性及び２５℃サイクル特性は比較例５に係る電池の値を１００とした場合の相対値であり、また、低温充電後の容量復帰率特性は比較例６に係る電池の値を１００とした場合の相対値である。
【００３２】
【表３】

【００３３】
表３及び図３に示すように、Ｔｉ、Ｆ添加ＬｉＣｏＯ_２とＴｉ、Ｆ無添加ＬｉＣｏＯ_２の混合比が９．５：０．５〜６．５：３．５の範囲にあるときに、低温充電後容量復帰率は、無添加ＬｉＣｏＯ_２を混合しないものよりも向上した。なお、混合比を９．５：０．５としたときには、低温充電後容量復帰率の低下が目立ち、混合比を６．５：３．５としたときには、負荷特性の低下が目立った。従って、Ｔｉ、Ｆ添加ＬｉＣｏＯ_２とＴｉ、Ｆ無添加ＬｉＣｏＯ_２混合比は、好ましくは９．５：０．５〜６．５：３．５、さらに好ましくは９：１〜７：３の範囲である。
【００３４】
以上の実施例１〜１５及び比較例１〜６の結果を総合して判断すると、正極活物質としてコバルト酸リチウム、負極活物質として炭素を用いた非水電解質二次電池において、該コバルト酸リチウムとして、異種元素が添加されたコバルト酸リチウム及び異種元素が添加されていないコバルト酸リチウムの混合物からなるものを用いると、異種元素添加コバルト酸リチウムのみを使用した場合の低温充電後の容量復帰性能の低下を改良し得ることは明らかであり、この場合、前記異種元素が添加されたコバルト酸リチウム及び異種元素が添加されていないコバルト酸リチウムの混合比が質量比で９．５：０．５〜６．５：３．５、より好ましくは９：１〜７：３の範囲の場合に特に優れた低温充電後の容量復帰性能性と、負荷特性及びサイクル特性に優れたリチウム非水電解質二次電池が得られることがわかる。
【００３５】
なお、異種元素添加ＬｉＣｏＯ_２に添加される異種元素添加量は、ＺｒではＬｉＣｏＯ_２１ｍｏｌに対して、０．０１〜０．９ｍｏｌ％（９３〜８３２３ｐｐｍ）、Ｔｉでは０．０１〜０．５ｍｏｌ％（４９〜２４４３ｐｐｍ）、Ｆでは０．００３６〜２７ｍｏｌ％（７〜５００００ｐｐｍ）の範囲であることが好ましい。これは、上述の範囲より少ない添加量では、異種元素を添加することによる電池負荷特性向上等の効果が小さく、上述範囲より添加量が多いと、電池容量が低下するためである。
【００３６】
【発明の効果】
以上述べたとおり、本発明によれば、低温充電後の容量復帰性能性と、負荷特性及びサイクル特性に優れたリチウム非水電解質二次電池が得られる。
【図面の簡単な説明】
【図１】図１は、Ｚｒ添加ＬｉＣｏＯ_２混合比と電池特性の変化を表す図である。
【図２】図１は、Ｆ添加ＬｉＣｏＯ_２混合比と電池特性の変化を表す図である。
【図３】図１は、Ｔｉ、Ｆ添加ＬｉＣｏＯ_２混合比と電池特性の変化を表す図である。[0001]
The present invention relates to a non-aqueous electrolyte secondary battery using lithium cobaltate (LiCoO ₂ ) as a positive electrode material, and particularly to a reduction in capacity recovery performance after low-temperature charging when only lithium cobaltate with a different element is used. The present invention relates to a non-aqueous electrolyte secondary battery having improved characteristics.
[0002]
[Prior art]
With the rapid spread of portable electronic devices, the required specifications for batteries used in them have become stricter year by year. Particularly, small and thin type, high capacity, excellent cycle characteristics, and stable performance are required. I have. In the field of secondary batteries, lithium non-aqueous electrolyte secondary batteries, which have a higher energy density than other batteries, are attracting attention, and the proportion of lithium non-aqueous electrolyte secondary batteries has shown a significant growth in the secondary battery market. ing.
[0003]
The lithium non-aqueous electrolyte secondary battery includes a negative electrode in which a negative electrode mixture containing a negative electrode active material that inserts and desorbs lithium ions is applied to both surfaces of a negative electrode core (current collector) formed of a long and thin sheet-like copper foil, A separator made of a microporous polypropylene film or the like is placed between a positive electrode coated with a positive electrode mixture containing a positive electrode active material that absorbs and releases lithium ions on both sides of a positive electrode core made of an elongated sheet-like aluminum foil etc. Then, after the negative electrode and the positive electrode are wound in a columnar or elliptical shape in a state where they are insulated from each other by a separator, the negative electrode tab and the positive electrode tab are respectively connected to predetermined portions of the negative electrode and the positive electrode, and are crushed and flattened as necessary. The wound electrode body is manufactured by covering the outside with an exterior.
[0004]
As a positive electrode material, lithium cobalt oxide (LiCoO ₂ ) is conventionally used mainly because it exhibits a potential of 4 V or more with respect to lithium and can realize a secondary battery having a high energy density. However, cobalt is expensive. In addition, because of its small amount as a resource, in order to continue using this lithium cobalt oxide as a positive electrode material of a non-aqueous electrolyte secondary battery, further enhancement of the performance and life of the non-aqueous electrolyte secondary battery is desired. In particular, it is desired to improve load characteristics and cycle characteristics.
[0005]
On the other hand, as a method for improving characteristics of a lithium nonaqueous electrolyte secondary battery using lithium cobaltate as a positive electrode active material, a method of adding a different element to lithium cobaltate is known. For example, Patent Literature 1 below discloses a nonaqueous electrolyte secondary battery that generates a high voltage by adding zirconium to lithium cobalt oxide, which is a positive electrode active material, and exhibits excellent charge / discharge characteristics and storage characteristics. ing.
[0006]
This zirconium-added lithium cobalt oxide is stabilized by covering the surface of the LiCoO ₂ particles with zirconium oxide ZrO ₂ or a composite oxide of lithium and zirconium Li ₂ ZrO ₃ , and as a result, even at a high electric potential, This is because a positive electrode active material exhibiting excellent cycle characteristics and storage characteristics can be obtained without causing a decomposition reaction or crystal destruction of the liquid, and this effect is obtained by simply adding a zirconium or zirconium compound to LiCoO ₂ after firing. Can be obtained only by mixing zirconium into a mixture of a lithium salt and a cobalt compound, followed by firing.
[0007]
[Patent Document 1]
JP-A-4-319260 (claims, paragraphs [0006], [0008] to [0011])
[0008]
[Problems to be solved by the invention]
The present inventors have already used a lithium nonaqueous electrolyte by using at least one of the different kinds of elements including not only zirconium (Zr) but also titanium (Ti) and fluorine (F) as the positive electrode active material. It has been found that the load characteristics and the cycle characteristics of the secondary battery can be improved, but only lithium cobaltate to which at least one selected from the group consisting of Zr, Ti, and F is added as a different element is used as the positive electrode active material. Lithium non-aqueous electrolyte secondary batteries have a problem that the capacity recovery performance after low-temperature charging is simultaneously reduced. Such a problem is a phenomenon that does not occur in cobalt lithium oxide to which no different element is added.
[0009]
Therefore, the inventors have conducted various experiments and found that, when lithium cobaltate with the addition of a different element and lithium cobaltate without the addition of a different element are used in combination, low-temperature charging when only the lithium cobaltate with a different element is used as the positive electrode active material The disadvantage that the capacity recovery performance decreases afterwards is compensated for by lithium cobalt oxide without the addition of a different element, and the disadvantage that the lithium cobalt oxide without the addition of a different element is inferior in load characteristics and cycle characteristics is due to lithium cobalt oxide with a different element added. They found out that the advantages of both were effectively and synergistically exhibited, and completed the present invention.
[0010]
That is, an object of the present invention is to provide a non-aqueous electrolyte secondary battery in which a decrease in capacity recovery performance after low-temperature charging when only a different element-added lithium cobalt oxide is used is improved.
[0011]
[Means for Solving the Problems]
The above object of the present invention can be solved by the following aspects. According to one embodiment of the present invention, in a nonaqueous electrolyte secondary battery using lithium cobaltate as a positive electrode active material and carbon as a negative electrode active material, the lithium cobaltate includes lithium cobaltate to which a different element is added, Provided is a non-aqueous electrolyte secondary battery using a mixture of lithium cobalt oxide to which no element is added.
[0012]
According to this aspect, the disadvantage that the capacity recovery performance is reduced after low-temperature charging when only the different element-added lithium cobalt oxide is used as the positive electrode active material is compensated for by the lithium cobalt oxide to which the different element is not added. The disadvantage that lithium cobalt oxide without addition of lithium is inferior in load characteristics and cycle characteristics is compensated for by lithium cobalt oxide added with a different element, and the advantages of both are synergistically exhibited.
[0013]
In this embodiment, the mixing ratio of the lithium cobalt oxide to which the different element is added and the lithium cobalt oxide to which the different element is not added is 9.5: 0.5 to 6.5: 3.5 by mass ratio. Preferably, the ratio is 9: 1 to 7: 3. If the proportion of the different element-added lithium cobalt oxide exceeds 95 parts by mass, the load characteristics and the cycle characteristics are good, but the capacity recovery rate after low-temperature charging is undesirably reduced. On the other hand, if the proportion of the lithium cobalt oxide containing the different element is less than 65 parts by weight, the capacity return rate after low-temperature charging becomes good, but the load characteristics and the cycle characteristics deteriorate.
[0014]
Further, according to this aspect, it is preferable that the different element is at least one selected from Zr, Ti, and F. The use of different element-added lithium cobalt oxide containing these elements makes it possible to effectively improve the load characteristics and cycle characteristics of a non-aqueous electrolyte secondary battery using lithium cobalt oxide as a positive electrode active material. .
[0015]
[Action]
According to the results of studies by the present inventors, the reason that the above aspect solves the problem that the capacity recovery rate after low-temperature charging when only the different element-added lithium cobalt oxide is used as the positive electrode active material is reduced. Seems to be as follows.
[0016]
That is, it is generally known that when a lithium nonaqueous electrolyte secondary battery is charged at a low temperature, the subsequent decrease in battery capacity is greater than when the battery is charged at room temperature. This is because, in a lithium nonaqueous electrolyte secondary battery, a reaction occurs in which lithium ions are electrochemically desorbed from lithium cobalt oxide of the positive electrode and inserted into carbon of the negative electrode as the battery is charged. When the battery is charged at a temperature extremely lower than that, a part of the lithium ions desorbed from the positive electrode is deposited on the negative electrode as lithium metal, and cannot participate in the subsequent charge / discharge reaction. The reason why lithium metal is deposited on the negative electrode at low temperature is that the polarization of the negative electrode becomes larger at low temperature charging than at room temperature charging, the potential of the negative electrode drops to the potential at which lithium metal is deposited, and the amount of lithium metal deposited is This is presumed to be determined by the degree of polarization of the negative electrode during the charging reaction.
[0017]
When lithium cobaltate to which a different element is added is used for the positive electrode, the capacity recovery performance after low-temperature charging is lower than when lithium cobaltate to which no different element is added is used for the positive electrode. Therefore, the polarization of the negative electrode is strongly affected by the positive electrode material, and the polarization of the negative electrode at a low temperature is higher when the lithium cobaltate without the addition of a different element is used as the positive electrode than when the lithium cobaltate with the addition of a different element is used for the positive electrode. It is estimated to be larger than when used for
[0018]
When 5 to 35 parts by mass, more preferably 10 to 30 parts by mass of lithium cobaltate without the addition of a different element is mixed with lithium cobaltate with the addition of a different element, the mixture of the lithium cobaltate without the addition of a different element allows the negative electrode to be mixed. It is presumed that the polarization at low temperature was relaxed, the amount of lithium deposited during low-temperature charging was reduced, and as a result, the capacity recovery rate after low-temperature charging was improved. When the mixing ratio of lithium cobalt oxide without addition is less than 5 parts by mass, the capacity recovery performance after low-temperature charging did not improve, presumably because polarization was not sufficiently relaxed.
[0019]
On the other hand, when the mixing ratio of the lithium cobalt oxide containing no different element exceeds 35 parts by mass, the capacity return performance after low-temperature charging was not significantly improved, but the load characteristics were reduced. This is presumably because if the mixing ratio of lithium cobaltate containing no foreign element exceeds 35 parts by mass, a sufficient effect of adding the foreign element cannot be obtained. Therefore, in the present invention, in order to maintain the load characteristics and cycle characteristics and to improve the capacity recovery performance after low-temperature charging, lithium cobalt oxide to which a different element is added and lithium cobalt oxide to which no different element is added are used. It was concluded that it was necessary to mix the mixture in a mass ratio of 9.5: 0.5 to 6.5: 3.5, preferably 9: 1 to 7: 3.
[0020]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, specific examples of the present invention will be described in detail with reference to Examples and Comparative Examples.
(Examples 1 to 5, Comparative Examples 1 and 2)
[Preparation of positive electrode]
As a starting material, lithium carbonate (Li ₂ CO ₃ ) was used as a lithium source. As a cobalt source, cobalt tetroxide (Co ₃ O ₄ ) to which zirconium (Zr) was added as a different element during the synthesis of cobalt carbonate so as to be 1900 ppm in mass ratio to the total amount of the positive electrode active material was used. These were weighed so that the molar ratio of Li / Co became 1, then mixed in a mortar, and fired at 850 ° C. for 20 hours in an air atmosphere to obtain Zr-added lithium cobalt oxide (LiCoO ₂ ). This was ground in a mortar to an average particle size of about 10 μm to obtain a positive electrode active material. For the synthesis of LiCoO ₂ without addition of a different element, it was prepared in the same manner as Zr-added LiCoO ₂ except that Co ₃ O ₄ without addition of Zr was used as a cobalt source. The composition of the positive electrode active material was analyzed by a plasma emission analysis (Inductive Coupled Plasma: ICP).
[0021]
The Zr-added LiCoO ₂ powder and the non-added LiCoO ₂ powder are mixed at a predetermined mixing ratio, the mixed LiCoO ₂ powder is 85 parts by mass, the carbon powder as a conductive agent is 10 parts by mass, and polyvinylidene fluoride as a binder is used. The powder was mixed so as to be 5 parts by mass, and this was mixed with an N-methylpyrrolidone (NMP) solution to prepare a slurry. This slurry was applied to both sides of a 20 μm-thick aluminum current collector by a doctor blade method to form an active material layer, and then compressed to 170 μm using a compression roller. A positive electrode having a side length of 500 mm was produced.
[0022]
[Preparation of negative electrode]
95 parts by mass of the natural graphite powder and 5 parts by mass of the polyvinylidene fluoride powder were mixed, and this was mixed with an NMP solution to prepare a slurry. This slurry was coated on one side of an 18 μm-thick copper current collector. Was applied by a doctor blade method to form an active material layer. Then, it was compressed to 155 μm using a compression roller to produce a negative electrode having a short side of 57 mm and a long side of 500 mm.
[0023]
[Preparation of electrolyte solution]
1 mol / L of LiPF ₆ was dissolved in an equal volume mixed solvent of ethylene carbonate and dierucarbonate to prepare an electrolytic solution, which was used for producing a battery.
[0024]
[Measurement of load characteristics]
At 25 ° C., after constant-current charging (current 1500 mA, final voltage 4.2 V) -constant-voltage charging (voltage 4.2 V, final current 30 mA) -constant-current discharging (current 4500 mA, final voltage 2.75 V) at 25 ° C. Constant-current charging (current 1500 mA, final voltage 4.2 V) -constant-voltage charging (voltage 4.2 V, final current 30 mA) -constant-current discharging (current 4500 mA, final voltage 2.75 V). The load characteristics were evaluated by the ratio of the constant current discharge capacity of 4500 mA to the constant current discharge capacity of 1500 mA.
[0025]
[Measurement of capacity recovery rate after low-temperature charging]
At 25 ° C., each battery was subjected to constant-current charging (current 1500 mA, final voltage 4.2 V) -constant-voltage charging (voltage 4.2 V, final current 30 mA) -constant-current discharging (current 1500 mA, final voltage 2.75 V). Next, three cycles of constant current charging (current 1500 mA, final voltage 4.2 V) -constant voltage charging (voltage 4.2 V, final current 30 mA) -constant current discharging (current 1500 mA, final voltage 2.75 V) at 0 ° C. Repeated. Finally, a cycle of constant current charging (current 1500 mA, final voltage 4.2 V) -constant voltage charging (voltage 4.2 V, final current 30 mA) -constant current discharging (current 1500 mA, final voltage 2.75 V) at 25 ° C. . The capacity recovery rate after low-temperature charging was evaluated by the ratio of the discharge capacity after returning from 0 ° C. to 25 ° C. to the initial discharge capacity at 25 ° C.
[0026]
[Measurement of cycle capacity retention rate]
At a predetermined temperature of 25 ° C. or 45 ° C., each battery was charged with a constant current (current 1500 mA, end voltage 4.2 V) -constant voltage charge (voltage 4.2 V, end current 30 mA) -constant current discharge (current 1500 mA, end) Voltage 2.75 V). This was the first cycle. The ratio of the discharge capacity at the 500th cycle to the discharge capacity at the first cycle of each battery was evaluated as a capacity retention ratio.
[0027]
As Examples 1 to 5 and Comparative Examples 1 and ₂ , lithium non-aqueous electrolyte secondary batteries were prepared by varying the addition ratio of Zr-doped LiCoO ₂ and Zr-free LiCoO _2, and the load characteristics and the low-temperature Table 1 and FIG. 1 summarize the measurement results of the capacity return rate after charge and the cycle capacity retention rate at 25 ° C. Note that the load characteristics and the 25 ° C. cycle characteristics are relative values when the value of the battery according to Comparative Example 1 is set to 100, and the capacity return ratio characteristics after low-temperature charging are 100% as compared with the value of the battery according to Comparative Example 2. Is a relative value when
[Table 1]

[0028]
As shown in Table 1 and FIG. 1, when the mixture ratio of Zr-doped LiCoO ₂ and Zr-free LiCoO ₂ is in the range of 9.5: 0.5 to 6.5: 3.5, the capacity after low-temperature charging. The reversion rate was improved as compared with the case where no additive LiCoO ₂ was mixed. When the mixture ratio was 9.5: 0.5, the capacity return ratio after low-temperature charging was significantly reduced, and when the mixture ratio was 6.5: 3.5, the load characteristics were significantly reduced. Therefore, the mixing ratio of Zr-added LiCoO ₂ and Zr-free LiCoO ₂ is preferably in the range of 9.5: 0.5 to 6.5: 3.5, and more preferably in the range of 9: 1 to 7: 3.
[0029]
(Examples 6 to 10, Comparative Examples 3 and 4)
In Examples 6 to 10, fluorine (F) was used as a different element, and first, LiF was used in place of zirconium (Zr) as a different element in Examples 1 to 3, and the amount of fluorine (F) was 350 ppm. Batteries according to Examples 6 to 10 were produced using the different element-added LiCoO ₂ synthesized in the same manner as in Examples 1 to 3 except that the batteries were used as follows. As Comparative Examples 3 and 4, batteries of Comparative Examples 6 to 10 were produced by preparing LiCoO ₂ without addition of a different element prepared in the same manner as in Examples 5 to 8 except that LiF was not added. The load characteristics, the rate of capacity return after low-temperature charging, and the rate of cycle capacity maintenance at 45 ° C. were measured for each battery. The results are summarized in Table 2 and FIG. Note that the load characteristics and the 45 ° C. cycle characteristics are relative values when the value of the battery according to Comparative Example 3 is set to 100, and the capacity return ratio characteristics after low-temperature charging are 100% of the value of the battery according to Comparative Example 4. Is a relative value when
[Table 2]

[0030]
As shown in Table 2 and FIG. 2, when the mixing ratio of F-doped LiCoO ₂ and F-doped LiCoO ₂ is in the range of 9.5: 0.5 to 6.5: 3.5, the capacity after low-temperature charging. The reversion rate was improved as compared with the case where no additive LiCoO ₂ was mixed. When the mixture ratio was 9.5: 0.5, the capacity return ratio after low-temperature charging was significantly reduced, and when the mixture ratio was 6.5: 3.5, the load characteristics were significantly reduced. Therefore, the mixing ratio of F-added LiCoO ₂ to F-free LiCoO ₂ is preferably in the range of 9.5: 0.5 to 6.5: 3.5, and more preferably in the range of 9: 1 to 7: 3.
[0031]
(Examples 11 to 15, Comparative Examples 5 to 6)
In Examples 11 to 15, titanium (Ti) and fluorine (F) were simultaneously used as different elements, and first, LiF was replaced with fluorine (F) in place of zirconium (Zr) as the different element in Examples 1 to 3. The amount of the different element-added LiCoO ₂ synthesized in the same manner as in Examples 1 to 3 except that TiO ₂ was used so that the amount became 350 ppm and the amount of titanium (Ti) became 800 ppm was used. Such a battery was manufactured. As Comparative Examples 5 to 6, LiCoO ₂ containing no different element was prepared in the same manner as in Examples 11 to 15 except that TiO ₂ and LiF were not added, and batteries of Comparative Examples 5 to 6 were prepared. The load characteristics, the rate of capacity return after low-temperature charging, and the rate of cycle capacity maintenance at 25 ° C. were measured for each battery. The results are summarized in Table 3 and FIG. Note that the load characteristics and the 25 ° C. cycle characteristics are relative values when the value of the battery according to Comparative Example 5 is set to 100, and the capacity return ratio characteristics after low-temperature charging are 100% of the value of the battery according to Comparative Example 6. Is a relative value when
[0032]
[Table 3]

[0033]
As shown in Table 3 and FIG. 3, when the mixing ratio of Ti, F-added LiCoO ₂ and Ti, F-free LiCoO ₂ is in the range of 9.5: 0.5 to 6.5: 3.5, The capacity recovery rate after low-temperature charging was improved as compared with the case where no added LiCoO ₂ was mixed. When the mixture ratio was 9.5: 0.5, the capacity return ratio after low-temperature charging was significantly reduced, and when the mixture ratio was 6.5: 3.5, the load characteristics were significantly reduced. Thus, Ti, F added LiCoO ₂ and Ti, F no addition LiCoO ₂ mixing ratio is preferably 9.5: 0.5 to 6.5: 3.5, more preferably from 9: 1 to 7: 3 in the range It is.
[0034]
Judging from the results of Examples 1 to 15 and Comparative Examples 1 to 6, the lithium cobalt oxide was used in a nonaqueous electrolyte secondary battery using lithium cobalt oxide as a positive electrode active material and carbon as a negative electrode active material. When a mixture of lithium cobalt oxide to which a different element is added and lithium cobalt oxide to which a different element is not added is used, the capacity recovery performance after low-temperature charging when only a different element added lithium cobalt oxide is used In this case, it is clear that the mixing ratio of the lithium cobalt oxide to which the different element is added and the lithium cobalt oxide to which the different element is not added is 9.5: 0.5 in mass ratio. To 6.5: 3.5, more preferably 9: 1 to 7: 3, particularly excellent capacity recovery performance after low-temperature charging, load characteristics and cycle. It can be seen that excellent lithium non-aqueous electrolyte secondary battery sexual obtained.
[0035]
Incidentally, the addition amount heterogeneous element added to the different element added LiCoO _2, relative to Zr in _{LiCoO 2 1mol, 0.01~0.9mol% (93~8323ppm} ), Ti in 0.01 to 0.5 mol% (49 to 2443 ppm), and the content of F is preferably in the range of 0.0036 to 27 mol% (7 to 50,000 ppm). This is because if the amount of addition is smaller than the above range, the effect of adding a different element, such as improvement in battery load characteristics, is small, and if the addition amount is larger than the above range, the battery capacity decreases.
[0036]
【The invention's effect】
As described above, according to the present invention, a lithium nonaqueous electrolyte secondary battery having excellent capacity return performance after low-temperature charging, and excellent load characteristics and cycle characteristics can be obtained.
[Brief description of the drawings]
FIG. 1 is a diagram showing a change in a Zr-added LiCoO ₂ mixing ratio and battery characteristics.
FIG. 1 is a diagram showing a change in a mixing ratio of F-added LiCoO ₂ and battery characteristics.
FIG. 1 is a diagram showing a change in a mixing ratio of Ti and F-added LiCoO ₂ and battery characteristics.

Claims

In a nonaqueous electrolyte secondary battery using lithium cobaltate as a positive electrode active material and carbon as a negative electrode active material, lithium cobaltate to which a different element is added and lithium cobaltate to which no different element is added as the lithium cobaltate A non-aqueous electrolyte secondary battery characterized by using a mixture comprising:

A mixing ratio of the lithium cobalt oxide to which the different element is added and the lithium cobalt oxide to which the different element is not added is 9.5: 0.5 to 6.5: 3.5 by mass ratio. The non-aqueous electrolyte secondary battery according to claim 1.

3. The non-aqueous solution according to claim 2, wherein a mixing ratio of the lithium cobalt oxide to which the different element is added and the lithium cobalt oxide to which the different element is not added is 9: 1 to 7: 3 by mass ratio. 4. Electrolyte secondary battery.

4. The non-aqueous electrolyte secondary battery according to claim 1, wherein the different element is at least one selected from Zr, Ti, and F. 5.